Another Case:
Incase you encountered the error below.
"W: GPG error: http://www.debian-multimedia.org testing Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 07DC563D1F41B907"

Fix:
$apt-get install debian-multimedia-keyring

3. You can now execute your update
$apt-get update

If it doesn't work try this!

rm /var/lib/apt/lists/* -vf
apt-get clean
apt-get update

Another Case:

W: GPG error: ftp://ftp.deb-multimedia.org squeeze Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 07DC563D1F41B907
W: GPG error: ftp://ftp.deb-multimedia.org squeeze-backports Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 07DC563D1F41B907